From CH 469 810, a thin-wall steel product in the form of a sheet or strip and a method for its production are known, which can be used for the production of tinplate with a higher strength. The steel product is produced from an unalloyed steel with a carbon content of 0.03-0.25 wt % and has a manganese content of 0.2-0.6 wt % and a silicon content of less than 0.011 wt %. The steel product is characterized by a fine structure, consisting, at least partially, of martensite and ferrite, and has tensile strengths of at least 6328 kg/cm2 and an elongation at break of at least 1.5%. For the formation of these characteristics, the steel product is first heated in a furnace to a temperature above the A1 point and subsequently quenched in a water bath.
Increasingly, higher demands are made on the characteristics of metallic materials for the production of packagings, in particular with regard to their formability and their strength and their corrosion resistance. It is true that so-called dual phase steels are known from the automobile industry, which have a multiphase structure, which essentially consists of martensite and ferrite or bainite, and which, on the one hand, have a high tensile strength and, on the other hand, a high elongation at break also. Such a dual phase steel with a yield strength of at least 580 MPa and an elongation at break A80 of at least 10% is known, for example, from WO 2009/021898 A1. As a result of the combination of the material characteristics of such dual phase steels with a high strength and a good deformability, these dual phase steels are suitable, in particular, for the production of complex-shaped and highly stressable components, as are needed, for example, in the area of body construction for automobiles.
The alloy of the known dual phase steels is, as a rule, composed of a martensite fraction of 20% to 70% and any residual austenite fraction and ferrite and/or bainite. The good formability of dual phase steels is guaranteed by a relatively soft ferrite phase and the high strength is produced by the solid martensite and bainite phase, bound in a ferrite matrix. The desired characteristics with regard to formability and strength can be controlled in dual phase steels, within broad limits, by the alloy composition. Thus, for example, by the addition of silicon, the strength can be increased by the hardening of the ferrite or the bainite. By the addition of manganese, the martensite formation can be influenced positively and the formation of perlite can be prevented. Also, the addition of aluminum, titanium, and boron can increase the strength. The addition of aluminum is, moreover, utilized for the deoxidation and the binding of any nitrogen contained in the steel. For the formation of the multiphase alloy structure, dual phase steels are subjected to a recrystallizing (or austenitizing) heat treatment, in which the steel strip is heated to such temperatures, with subsequent cooling, that the desired multiphase alloy structure is established with an essentially ferritic-martensitic structure formation. Usually, cold-rolled steel strips are annealed in a recrystallizing manner in a throughput annealing process in an annealing furnace for economic reasons, wherein the parameters of the annealing furnace, such as through-flow speed, annealing temperature, and cooling rate, are established in accordance with the required structure and the desired material characteristics.
From DE 10 2006 054 300 A1, a higher-strength dual phase steel and a method for its production are known, wherein in the production method, a cold- or hot-rolled steel strip is subjected to a recrystallizing through-flow annealing in a through-flow annealing furnace, in a temperature range of 820° C. to 1000° C., and the annealed steel strip is subsequently cooled from this annealing temperature, at a cooling rate between 15 and 30° C. per second.
As a rule, the dual phase steels known from the automobile industry are not suitable for use as packaging steel, because especially due to the high fractions of alloy elements, such as manganese, silicon, chromium, and aluminum, they are very expensive and because some of the known alloy elements should not be employed for use as packaging steel in the food area, because a contamination of the food by diffusion of the alloy components into the contents must be ruled out. Furthermore, many of the known dual phase steels have such a high strength that they cannot be cold-rolled with the units usually used for the production of packaging steel.
Packaging steel must, moreover, have a high corrosion resistance and a good resistance to acids, since the contents of the packagings made of packaging steel, such as cans for beverages and food, frequently contain acid. Packaging steel, therefore, has a metallic coating as an anti-corrosion layer. The quality of this anti-corrosion layer depends, very substantially, on its adhesive capacity to the steel sheet surface. To improve the corrosion resistance of the coating and the adhesion of the anti-corrosion layer on the steel sheet surface, the tin coating placed galvanically on the steel sheet, for example, during the production of tinplate, is melted after the coating process. To this end, the coating deposited galvanically on the steel strip is heated to a temperature slightly above the melting point of the coating material (with a tin coating, for example, to 240° C.) and is subsequently quenched in a water bath. By the melting of the coating, the surface of the coating receives a shiny appearance and the porosity of the iron-tin alloy layer between the coating and the steel sheet is reduced, wherein its corrosion resistance is increased and its permeability for aggressive substances, for example, organic acids, is reduced.